The present invention relates brushless permanent magnet motors. More specifically, the invention relates to interactive sensing methods and apparatus employing the third harmonic component of the stator voltage signals of such motors to control operation of such motors.
A brushless permanent magnet (BPM) motor, also referred to as a permanent magnet brushless direct current (PMBDC) motor, a permanent magnet self-synchronous motor or an electronically commutated motor, is a type of motor that comprises a multi full pitch concentrated winding stator configuration with different possibilities for the number of phases and poles, and a rotor that has permanent magnets mounted in a magnetic structure attached to the motor shaft. The magnets can be either mounted on the surface of the rotor structure (surface mounted or inset permanent magnet motor) or inside it (buried or interior permanent magnet motor). The BPM motor is driven or operated by controlled application of current signals to the stator windings.
During operation, the rotor magnets produce an air gap flux density distribution that is a function of the type of their magnetization characteristic and fabrication process. When the magnets are magnetized axially, a trapezoidal air gap flux density is produced. When this magnetization is parallel to the magnet main axis a sinusoidal air gap flux density is generated. Because the main flux is produced by magnets that do not carry currents, motor losses occur that are restricted to the copper and iron losses in the stator and to iron loss in the rotor. Hence, a BPM motor is suitable for applications where high efficiency is a concern.
Due to their high efficiency and relative control simplicity, BPM motors are becoming preferred in appliance applications such as compressors, fans, pumps, and washers. Yet, in order to operate a BPM motor adequately, information about the position of the rotor is necessary. This information is used to define stator currents which are applied by an inverter so that the flux produced by these currents is always kept in quadrature with the rotor flux. This allows a complete decoupling between rotor flux and stator current vectors, and the result is a motor that has speed and torque proportional to the voltage and current amplitude, respectively, similarly to a direct current (DC) motor.
It is possible to sense the back electromotive force (EMF) of a motor to estimate the position of the rotor. However, the back EMF signal cycles only once per revolution of the rotor producing only two zero crossings per cycle and thus is not entirely suitable for controlling stator currents that must be defined three times more often during a revolution for a three-phase motor because the rotor position can only be estimated twice per revolution. Moreover, back EMF signals can be noisy, and filters therefor can introduce delay.
The general practice is to calibrate operation of a BPM motor for efficiency at one speed. Usually this is accomplished by detecting zero crossings of the back EMF signal and then gating current application based on preselected delays, the delays accommodated efficient operation at one speed. But at other speeds, the delays are not entirely suitable. Thus, the BPM motor operates inefficiently at other speeds.
In FIG. 1 there is illustrated the idealized air gap flux density distribution in a BPM motor with magnets radially magnetized. It is illustrated that the resultant trapezoidal air gap flux density has a dominant third harmonic component that links the stator windings inducing a third harmonic voltage component in each one of the phases. Other high frequency components such as 5.sup.th, 7.sup.th and 11.sup.th harmonics, and a switching frequency with its side bands, are also present in the air gap flux, but they are weak relative to the third harmonic and thus the third harmonic is the dominant component.
In a three-phase system, all third harmonic voltage components are in phase, forming a zero sequence set. A third harmonic voltage component is induced in the stator phases and corresponds exactly to the air gap third harmonic component because no third harmonic currents can circulate in star connected stator windings.
It can be appreciated that a summation of the three stator phase voltages results in the elimination of all polyphase components like the fundamental and characteristic harmonics. Only the third harmonic, and other triplens together with the PWM switching frequency and its side bands will be present in the adder output signal, the third harmonic being the dominant component. The result is a signal that can be used to identify rotor position that cycles three times per rotor revolution, and this provides more accurate rotor positional information than does only a back EMF signal.
Further background information regarding BPM motors and means and methods for obtaining the third harmonic signal are described in the following United States patents, the disclosures of which are incorporated herein by reference:
4,481,440 PA1 4,959,596 PA1 4,296,362 PA1 4,585,982 PA1 4,585,983 PA1 4,641,066 PA1 5,023,924 PA1 4,980,617 PA1 4,912,378 PA1 4,922,169
U.S. Pat. Nos. 4,912,378 and 4,641,066, in particular, provide excellent background discussions.
One concern with the summation of the stator phase voltages as described above, is that access to the neutral point connection or node of the stator is necessary. For this purpose, a wire connection to the neutral node, and although easy to install in the majority of applications, it can, in some cases, represent extra cost or inconvenience to the installation.
Another problem can arise when a BPM motor is operated at high torque or high speeds if back EMF sensing is needed by the motor. At high torque or high speeds, the back EMF no longer is available due to blanking out by the commutation of the invertor.